Having learned some valuable lessons from trying to drive our SGVHAK rover using an interface based on polar coordinates, we returned to the drawing board to create an alternative. Thankfully, the abstracted command structure meant we could experiment with front-end user interface without any changes to the back-end rover calculations.

Our first-hand experience taught us polar coordinate math are more trouble than it was worth, so our next attempt will stay in the Cartesian coordinate space. Furthermore, we’ll rearrange the layout so a position going full speed ahead would never be adjacent to a position going full speed reverse. (This was the case for polar coordinate system when doing full deflection turns.) Third, we’ll have to make sure low speed maneuvering has just as much room to fine-tune its angle as going high speed. And lastly, when someone releases their finger out of caution, we need to stop the rover but we don’t need to reset steering angles.

It turned out to be very easy to satisfy all of the above desires: map X-axis left-right dimension to steering angle, and Y-axis up-down dimension to speed.

Full speed ahead is now along the top edge of the blue rectangle, which is far from the bottom edge representing full speed reverse. There is no longer any risk of suddenly jerking between them.

At any given speed, we now have the full width for designating steering angle, we are no longer frustrated by a tiny range of adjustments when moving slow speed versus high.

And finally, when the user releases their finger, the dot will snap to vertical center (stop moving) but will remain at its most recent horizontal position, preserving the steering angle most recently used by our rover driver. This makes fine position adjustments much easier by allowing slow incremental inching forward/reverse movements.

Most users who tried both UI design prefer the Cartesian coordinate system. It was used for most of our driving at SCaLE 16X.

Once an abstraction layer was defined for communication between user interface and rover chassis calculations, I could start trying out various ideas for building a rover driving UI. The baseline rover UI presents two touchscreen controls mimicking joysticks on a traditional remote control vehicle: one moves up/down for controlling forward/back movement, and the other moves left/right for controlling steering. Driving the baseline rover using its UI is a two-thumb affair.

That is a perfectly valid and functional UI, but my ambitions grew beyond copying it. I wanted to explore ideas around single-finger operation. This decision held firm even after the person who created the UI for baseline rover warned us that it could be tricky to implement – single-point operation was already tried before going with the two-thumb approach. I may yet bow to their wisdom, but I wanted to give it a shot first!

The first implementation of a one-finger driving control is based around polar coordinates. It seemed like an obvious match to rover mechanics: polar coordinates on a two-dimensional plane is an angle and a distance, which maps directly to rover control’s angle and velocity. The user puts their finger down on the red central control dot, and they can drag it around inside the blue circle to drive the rover. Move the dot up to drive forward, rotate the dot left to steer left, and so forth.

As I got into it, though, problems started to mount.

The first and most obvious problem is that our underlying infrastructure – HTML, <canvas> tag, and JavaScript input events – use cartesian coordinates. In order to implement polar coordinate control we need trigonometry math to convert between the two systems. Errors creep in as we perform multiple operations and conversions between coordinate spaces.

The second problem was a consequence of mapping upper semicircle to forward velocity and lower semicircle to moving backwards. This became problematic when the user wishes to steer at maximum angle.  Turning far left means moving straight left, but at that point small movements can flip between upper and lower semicircle, causing the rover to jerk suddenly between forward and backward movement.

To mitigate this, a band of dead space between the semicircles was introduced. The user is no longer able to use the full 180 degrees of a semicircle for movement, they are restricted to the middle 140 degrees with the far left 20 and far right 20 degrees blocked off. This was effective to prevent sudden transitions between full speed forward/backwards motion but it wasted screen real estate.

The third problem of polar coordinates was exposed when trying to maneuver the rover at low-speed around obstacles. When our finger is near the center of the circle, small left-right movement translates into large changes in steering angle. It was more difficult than it should be to make small steering changes at low speed.

The last problem is makes the above problem worse. When somebody runs into a problem, their first instinct is to lift their finger off the touchscreen. In order to retreat to a known safe condition, our control dot pops back to the center position representing wheels stopped and all steerable wheels pointing straight front-back. This is fine, but when people are frustrated by problem #3 of low-speed maneuverability, this behavior snapping to straight front-back erases whatever angle they were previously driving at, making low-speed maneuvers even more frustrating.

And what happens when a rover driver is trying to perform delicate maneuvers but find themselves fighting a bad UI that gets in the way and cause frustration? The rover ends up running into things and risk breaking gearboxes.

Now we have experimental evidence polar coordinate driving doesn’t work as originally thought, let’s take that lesson and create a new UI.

Once we were confident our SGVHAK rover chassis control code looked pretty good, we were willing to trust that it could drive a rover without sending conflicting commands that make our rover try to twist itself into a pretzel. Now our focus turns to creating an interface for user control input.

A quick rough draft had been in place to help test rover chassis Ackermann math, letting a rover driver send a steering angle and travel speed to rover’s control code. But it was very tied to the current rover implementation: angle range was limited to angle range of what we have today, and speed was specified in raw quadrature pulses per second (QPPS) that we send straight down to RoboClaw motion controller‘s API.

Since we have ambitions for future rover variants, this direct dependency is not good. Future rovers may have a wider (or narrower) range of valid steering angles. They may also use something other than RoboClaw motor controllers. The answer is a typical software solution: abstraction!

This required changing the interface between our HTML UI front-end and the Flask-based Python code running on the rover. Instead of a fixed number angle or a fixed number of encoder pulses, user now send driving commands in terms of two percentages ranging from negative 100% to positive 100%.

For steering, -100% tells a rover to steer as far left as it could go, 0% is straight forward/back travel, and 100% is to steer as far right as it could go.

For velocity, -100% is travelling backwards as fast as it could, 0% is stop, and 100% is damn the torpedoes, full speed ahead.

Now when we adapt the code for future rover variants, we could focus on rover-side configuration and remain confident user-side HTML interface will automatically adapt to whatever capabilities are present on a particular rover variant.

This also allows us to experiment with user-side HTML without worrying about having to make matching changes in rover-side Python code. We can try different control schemes, as long as it ends up sending a velocity and steering angle percentage to the rover.

The first and crudest implementation of this concept is shown here: two HTML slider objects that move from -100% to 100%. The user can move the slider and tap “Send command” to rover. As the most basic HTML implementation possible, this is a useful test tool to isolate whether a particular control problem is in HTML front-end or in Python back-end, but it is obviously not a great driving interface.

That’s coming up next.

When we were putting together code to calculate Ackermann steering angles for SGVHAK rover‘s wheels, initial testing was done via Python command line using some fixed test values to match against results pre-calculated by hand. Once we built some confidence our math was on the right track, it was time to start testing against more varied inputs.

There was just one minor problem – at this point in rover development, our software is a tiny bit ahead of the hardware which means we can’t test on hardware that hasn’t yet been assembled. Even if we could, it wouldn’t be a great idea to jump from a small fixed set of test values to actual hardware. As an intermediate step, we created a bit of code to visualize results of roverchass.py calculations: the “Chassis Configuration” screen will show what our control code thinks all six wheels should be doing. (This shows the intent, which is different from hardware telemetry of what’s actually happening.)

Each of the six wheels got its own blue box, laid out roughly analogous to their physical position on the rover chassis. Originally wheel steering & speed was displayed as text: number of degrees a wheel should be steered, and a second number reflecting the intended velocity for each wheel. This was accurate information but it took some effort to interpret. To make this information easier to understand, numerical text was changed to a visual representation.

By using HTML’s <canvas> element, we could draw some simple graphics. Each wheel is represented by a black rectangular outline. A wheel’s steering angle is now represented by tilting its rectangle to that angle. Inside a wheel’s rectangle is a green fill representing desired velocity. When a wheel is supposed to be stopped, there is no green fill and its black rectangle is empty. When a wheel is supposed to be rolling forward, green rectangle fills from the middle upwards. Faster velocity is represented by more green in the upper half. For a wheel that should be rolling backwards, green fills downwards instead.

This visualization of roverchassis.py calculation results were far easier for humans to understand. This made it very quick to validate that roverchassis is behaving roughly correctly. There’s a place for precise engineering calculations, but there’s also place for a quick glance to check “yeah, that looks about right.”

One of the biggest changes in our new rover controller code is the goal of managing all ten rover chassis motors together, starting to think of the rover as a single unit instead of ten individual things.

This logic is centered in roverchassis.py, which is be responsible for calculating Ackermann angles for all steerable wheels in the rover, as well as calculating the desired travel velocity for each wheel. In order to make this code as flexible as we can, we offload details of rover chassis into configuration files and make our Python math as generic as possible running calculations based on configuration file values. This meant we could support rovers with different geometries without editing source code. This feature began with a desire to have flexible wheelbase and track dimensions of our rover chassis, but eventually extended to supporting similar but different configurations such as two-wheel differential drive.

Another goal was to support different types of motors and motor controllers. We appreciated all the features in the RoboClaw motor controller we started with, but we also thought about trading off some of these features for lower cost. In preparation for this flexibility, we kept roverchassis.py focused on general Ackermann-related math and kept motor-specific code in a separate class. In the first draft, this meant pulling all RoboClaw related code into roboclaw_wrapper.py which translates general concepts from roverchassis.py into specific commands in Ion Motion Control’s public domain API roboclaw.py. Later on we validated this flexibility by steering a single wheel with a RC servo followed by adding support for LewanSoul serial bus servos.

This was all possible using a flexible syntax in config_roverchassis.json that allows us to adjust wheel dimensions as well as switch out individual wheel travel or steering motors for different motor controllers.

When our SGVHAK rover project started moving beyond individual units of our custom wheel gearbox design and towards a multiple wheel chassis, it was happening on both the hardware and software fronts. Our Flask-based control UI has worked well up to this point, allowing us to tune RoboClaw motor controller parameters to suit our wheel design. Building this first draft UI taught us a lot, and as we learned we also realized how it reflected our ignorance at the beginning of this project.

And so we bid goodbye to our first draft rover UI RogerPiBot to start a new project SGVHAK_Rover. This way we retain all the testing & tuning capabilities of the existing software, leaving that as-is. Freeing us to create a new user experience.

The major changes are:

1. Simple touch-friendly controls instead of HTML forms with a lot of fields. The form was a very powerful tool for tuning parameters from a desktop workstation, but we’re getting out of the engineering test phase. All interfaces will now be tailored towards operation from a small touchscreen.
2. Rover as a unit instead of wheel as a unit. We need to start treating all six travel motors and four steering motors aboard our chassis as a single unit, as the user expects the software to do. Target audience for this software will not be interested in poke and prod at motors individually.
3. Configuration files instead of source code edits. The new software is built on the assumption that will be people who want to use it but is not familiar with Python or code in particular. Part of this meant configuration information will now be stored in human-readable JSON files at the root directory instead of asking people to go in and edit Python source code.

There were various other changes big and small. One of the larger changes was separate from the rover itself: we chose to switch away from using Bootstrap purely for the sake of HTML creation exercise. This time around we’re using a different library called Materialize to help create our responsive UI.

Our first attempt at rover software was a wonderfully informative hands-on adventure on creating a path from a cell phone’s web browser HTML all the way to serial bus commands sent to a RoboClaw motor controller. Fortunately or unfortunately, these lessons also put a finite lifespan to the software as we start to discover basic mistakes that we didn’t know we were making until we got a little smarter.

Before we leave the first draft behind, however, it had one final role to play. This point in software development timeline coincided with our rover wheel development’s completion. The actual six-wheeled rover chassis is still in the future, but our wheel development is mature enough for us to build a test chassis. Two wheel units were assembled and integrated into a bare-bones test chassis.

This chassis needed a brain and a controller.

To enable motor control via differential drive, we hacked together a minimal user interface for sending commands to our test chassis. Not much effort was spent on user-friendliness and its layout is very much an engineering number cruncher’s idea of an interface. (Also referred to as “bad UI.”) It was just enough to let us move the robot around and uncover some finishing issues in the wheel design, but not much more than that.

The robot moving under its own power attracted attention from other people in the work space. Naturally some asked if they could try driving the robot but one look at the interface and the response was “Um… maybe I’ll wait for a later version” and the phone was handed back.

No offense taken, it’s another vote in favor of moving on. We’ll close out the “RogerPiBot” project at this point and start a new rover control project incorporating all the lessons we’ve learned.